Page:EB1911 - Volume 06.djvu/862

having popularized the use of this invaluable combination.
The important point of his idea was that it combined steel and
concrete in such a way that the best qualities of each material
were brought into play. Concrete is readily procured and
easily moulded into shape.

Fig. 1.—Expanded Steel Concrete Slab.

It has considerable compressive
or crushing strength, but is somewhat deficient in shearing
strength, and distinctly weak in tensile or pulling strength.
Steel, on the other hand, is easily procurable in simple forms
such as long bars, and is exceedingly strong. But it is difficult
and expensive to work up into various forms. Concrete has been
avoided for making beams, slabs and thin walls, just because
its deficiency in tensile strength doomed it to failure in such
structures. But if a concrete slab be “reinforced” with a
network of small steel rods on its under surface where the
tensile stresses occur (see fig. 1) its strength will be enormously
increased. Thus the one point of weakness in the concrete
slab is overcome by the addition of steel in its simplest form,
and both materials are used to their best advantage. The
scientific and practical value of this idea was soon seized upon
by various inventors and others, and the number of patented
systems of combining steel with concrete is constantly increasing.
Many of them are but slight modifications of the older systems,
and no attempt will be made here to describe them in full.
In England it is customary to allow the patentee of one or other
system to furnish his own designs, but this is as much because he has
gained the experience needed for success as because of any special
virtue in this or that system. The majority of these systems have
emanated from France, where steel concrete is largely used. America
and Germany adopted them readily, and in England some very large
structures have been erected with this material.

Expanded Metal.

Section through Intersection.Fig. 2.

The concrete itself
should always be the very
best quality, and Portland
cement should be used on
account of its superiority
to all others. The aggregate should be the best obtainable and
of different sizes, the stones being freshly crushed and screened
to pass through a 7/8 in. ring. Very special care should be taken
so to proportion the sand as to make a perfectly impervious
mixture. The proportions generally used are 4 to 1 and 5 to 1
in the case of gravel concrete, or 1:2:4 or 1:2½:6 in the case
of broken stone concrete. But, generally speaking, in steel
concrete the cost of the cement is but a small item of the whole
expense, and it is worth while to be generous with it. If It is
used in piles or structures where it is likely to be bruised the
proportion of cement should be increased. The mixing and
laying should all be done very thoroughly; the concrete should
be rammed in position, and any old surface of concrete which has
to be covered should be cleaned and coated with fresh cement.

Fig. 3.—Hennebique System.

The reinforcement mostly consists of mild steel and sometimes
of wrought iron: steel, however, is stronger and
generally cheaper, so that in English practice it holds
the field. It should be mild and is usually specified to
have a breaking (tensile) strength of 28 to 32 tons per
sq. in., with an elongation of at least 20% in 8 in. Any
bar should be capable of being bent cold to the shape
of the letter U without breaking it. The steel is generally
used in the form of long bars of circular section. At
first it was feared that such bars would have a tendency
to slip through the concrete in which they were embedded,
but experiments have shown that if the bar
is not painted but has a natural rusty surface a very
considerable adhesion between the concrete and steel—as
much as 2 cwt. per sq. in. of contact surface—may
be relied upon. Many devices are used, however,
to ensure the adhesion between concrete and bar being
perfect. (1) In the Hennebique system of construction the
bars are flattened at the end and split to form a “fish tail.”
(2) In the Ransome system round bars are rejected in favour
of square bars, which have been twisted in a lathe in “barley
sugar” fashion. (3) In the Habrick system a flat bar similarly
twisted is used. (4) In the Thacher system a flat bar with
projections like rivet heads is specially rolled for this purpose.
(5) In the Kahn system a square bar with “branches” is used.
(6) In the “expanded metal” system no bars are used, but instead
a strong steel netting is manufactured in large sheets by special
machinery. It is made by cutting a series of long slots at regular
intervals in a plain steel plate, which is then forcibly stretched
out sideways until the slots become diamond-shaped openings,
and a trellis work of steel without any joints is the result
(fig. 2).

Fig. 4.
Hennebique System.

The structures in which steel concrete is used may be analysed
as consisting essentially of (1) walls, (2) columns, (3) piles, (4)
beams, (5) slabs, (6) arches. The designs
differ considerably according to which of
these purposes the structure is to fulfil.

The effect of reinforcing walls with steel is that they can be made much thinner.

The steel reinforcement is generally applied in the form of vertical rods built in the wall at intervals, with lighter horizontal rods which cross the vertical ones, and thus form a network of steel which is buried in the concrete. These rods assist in taking the weight, and the whole network binds the concrete together and prevents it from cracking under a heavy load. The vertical rods should not be quite in the middle of the wall but near the inner and outer faces alternately. Care must be taken, however, that all the rods are covered by at least an